The present invention relates to pumps and more particularly to an improved centrifugal pump for use with corrosive materials in which the pump chamber and impeller are fully lined with a polymeric plastic material such as perfluoroalkoxy (PFA) resin.
A wide variety of materials have been used in the past in attempts to protect pump parts which are in contact with corrosive liquids. Corrosion resistant metal alloys have been used but have had a number of drawbacks to their use including cost, difficulty of machining or casting parts, strength problems, and brittleness. Ceramics and glasses have been used as linings in pumps because of their inertness to most corrosive materials. However, they are quite brittle and susceptible to thermal and mechanical shock resulting in their failure.
Thermoplastic and thermosetting polymeric materials have also been used both as solid pump parts and as linings in pumps. Among these, the fluorocarbon polymers such as polytetrafluoroethylene (PTFE) and fluorinated ethylene-propylene copolymer (FEP) have found widespread use. For example, Wissman U.S. Pat. No. 3,551,067, assigned to the assignee of the present invention, discloses a centrifugal pump lined with PTFE which protects all exposed surfaces of the pump from corrosive materials. However, although PTFE possesses excellent corrosion resistance to a variety of materials over a wide range of temperatures, because of its exceptionally high melt viscosity, it cannot be processed by conventional extrusion or molding techniques. The processing steps required to produce a PTFE lined pump require significant amounts of both time and labor.
FEP has most of the desirable corrosion resistant properties of PTFE with the important advantage of being melt processable. That is, it can be processed by conventional thermoplastic techniques. However, its service temperature is less than that of PTFE. Perfluoroalkoxy (PFA) resin is similar to both PTFE and FEP in properties. Like FEP, it can be processed by conventional melt processing techniques. It does have better mechanical properties and dimensional stability than FEP at elevated service temperatures (i.e., temperatures above 150.degree. C.).
However, problems have been encountered in using such fluorocarbon polymers as linings in pumps because of the tendency of such polymers to cold flow under the hydraulic and clamping pressures and changes in temperature encountered during operation. Because of the close tolerances required between the pump casing and pumping element such as an impeller, such flow is undesirable and may cause failure of the pump. Addditionally, stresses are introduced into the lining during molding with additional stresses being caused by the high coefficient of expansion of the polymers in relation to the metal casing. These stresses in conjunction with the shrinkage which occurs in the lining after molding causes the lining to stress crack and fail during service. Previous attempts to restrain movement (cold flow) of the polymers during service such as by providing locking grooves have only contributed to the stress cracking problem.
Accordingly, the need exists in the art for a means of restraining undesirable movement of such polymer lining materials to preserve structural and dimensional integrity while dispersing the inherent stresses found in such linings and without causing their catastrophic failure (cracking).